This paper traces the history of Jung's ideas concerning the psychoid unconscious, from their origins in the work of the vitalist, Hans Driesch, and his concept of Das Psychoid, through the subsequent work of Eugen Bleuler, Director of the Burghölzli Asylum, and his concept of Die Psychoide, to the publication of Jung's paper On the Nature of the Psyche in 1947. This involves a review of Jung's early work and of his meeting with Freud, when apparently the two men discussed calling the unconscious ‘psychoid’, as well as a review of Jung's more mature ideas concerning a psychoid unconscious. I propose to argue that even at the time of their meeting, Jung had already formulated an epistemological approach that was significantly different from that of Freud and that clearly foreshadowed his later ideas as set out in On the Nature of the Psyche.